The invention relates to a method for producing workpieces from lightweight steel having material properties that can be adjusted across the wall thickness.
In the following, workpieces relate to components or primary products for components such as for example strips, plates or pipes which are used for example in the field of machine construction, plant construction, steel construction and ship construction, and in particular motor vehicle construction.
Especially the very competitive automobile market forces the manufacturers to constantly seek solutions for lowering fleet consumption while maintaining a maximal comfort and safety of occupants. In this context the weight saving of all vehicle components on one hand plays a decisive role, but on the other hand also properties of the individual components which promote the safety of the passengers in the case of high static and dynamic stresses during operation and in case of a crash.
In recent years great advances have been made in the field of the so called lightweight steels which are characterized by a low specific weight and at the same time high strengths and tenacities (for example EP 0 489 727 B1, EP 0 573 641 B1, DE 199 00199 A1) and a high ductility and with this are of significant interest for vehicle construction.
In these steels, which are austenitic in the initial state, a weight reduction which is advantageous for the automobile industry, is achieved while maintaining the previous construction method, as a result of the high proportion of alloy components which have a specific weight which is far below the specific weight of iron (Mn, Si, Al).
For example, from DE 10 2004 061 284 A1 a lightweight steel is known with an alloy composition (in weight %):
C0.04 to ≦1.0Al0.05 to <4.0Si0.05 to ≦6.0Mn 9.0 to <18.0
remainder iron including usual steel tramp elements. Optionally Cr, Cu, Ti, Zr, V and Nb can be added depending on the requirements.
This known lightweight steel has a partially stabilized γ-mixed crystal structure with a defined stacking fault energy with a at times multiple TRIP-effect which transforms the tension- or expansion-induced transformation of a face centered γ-mixed crystal (austenite) into an ε-martensite (hexagonally densest sphere packing) which then in the course of further deformation transforms into a body centered α-martensite and residual austenite.
The high degree of deformation is achieved by TRIP (transformation induced plasticity) and TWIP (twinning induced plasticity) properties of the steel.
Numerous tests have shown that in the complex interaction between Al, Si, Mn the carbon content is of paramount significance. On one hand it increases the stacking fault energy and on the other hand expands the meta-stable austenite region. This allows influencing the deformation-induced martensite formation and the strengthening and ductility associated therewith.
With these lightweight steels client specifications can already be met to a great extend, however there is still the demand for performance-optimized workpieces made of lightweight steel which have material properties with regard to strength, tenacity, wear resistance and so forth which are correspondingly adjusted to the stresses expected to occur in the direction of the wall or plate thickness. An example for this are bullet proof vehicles in which the component has to have a hard surface layer for fending off projectiles and a layer underneath this surface layer with a high tenacity for a high energy-absorption capacity in case of being fired on.
A method for producing a composite strip made of steel is for example known from DE 101 24 594 A1. According to this, a ferritic core strip which is directly cast according to the two-roll method and plated with an austenitic or high-alloyed ferritic cold strip.
Pipes with different material properties across the wall thickness are known inter alia from EP 0 944 443 B1. Here, a pipe is inserted into another pipe and connected with the other pipe, wherein different materials are used for the outer and inner pipes.
A disadvantage of these known methods is the sharp step of the properties of the composite material due to the plating which complicates the adjustment to the respective properties to the corresponding requirements across the wall or strip thickness and the high costs for the manufacture of the plating. In addition the weight advantage of the lightweight steels is mostly lost by the plating with conventional steels.
A further method for producing a composite material is known from DE 39 04 776 C2 in which several layers of steel are interconnected by means of diffusion welding and these layers are alloyed by means of metalloids in a gas atmosphere in such a manner that a different concentration profile of the metalloids is established across the cross section of the flat product.
This allows adjusting different material properties across the cross section of the composite material with regard to strength and tenacity.